Survival
of Phytophthora species and other pathogens in soilless media
components or soil and their eradication with aerated steam

Phytophthora ramorum, thought to be largely a
foliar pathogen on nursery crops, could be incorporated into container
soilless potting media and thus become soilborne. Also, nursery containers
from which infested media or infected plants were removed could be contaminated,
as is true with other soilborne fungal pathogens. Inoculum from either
source may initiate infections on subsequent crops grown in infested media
or contaminated containers. Pathogens could be eradicated from soilless
media by heat from steam, composting, or solarization; or by chemical
fumigation. Growers currently attempt to decontaminate used containers
by pressure washing and/or chemical sanitization. Many simply apply fungicides
during the production cycle to prevent infections or to respond to occurrence
of diseases.

P. ramorum was detected for 6 mo by B or DP from all substrates
amended with sporangia or chlamydospores in vermiculite, but was not detected
by either B or DF from infected leaf inoculum. P. ramorum sporangia
survived best in peatmoss, potting mix, coir, and DF bark, and poorest
in sand or soil. By comparison, P. cactorum was recovered after 5-6 mo
from infected leaf inoculum in all media. P. citricola was recovered
by B for only 3 mo in coir, potting mix, sand and soil, but not at all
from compost, DF bark, peatmoss, or RW sawdust. P. citrophthora, as with
P. ramorum, was never recovered from leaf inoculum in any material
at any time. All pathogens were killed by aerated steam treatments of
infested medium at 60º C or higher.

These results indicate (a) that P. ramorum
can survive very well in potting mix components or soil as culture-produced
sporangia or chlamydospores, but was not detected from infected leaf pieces
compared to other Phytophthora species that were, and (b) that
aerated steam pasteurization is an effective means of eradicating P.
ramorum as well as other pathogens from infested media and contaminated
containers without destroying the containers.